Sometimes someone will start a tag for a category that already has a tag. Stackexchange has a feature called tag synonyms that allow tags to be mapped onto other tags so that all variations on a single theme can be combined into one tag.
It seems someone accidentally started sefarim-books when we already had jewish-books. Since the latter came first, I suggested a tag synonym to map the former to the latter. Once it gets 4 votes by members who have at least a answer score of 5 in this tag, it will come into effect. You can vote on it here. (Incidentally, if it gets 2 downvotes it will be deleted, but I don't expect that to happen in this case.)
In general, tags are a useful way of sorting all the varied information on our site and proper tagging can help users searching for content and avoid duplicate questions. Especially as the site continues to grow, proper organization will become more important. Now there is some debate about different tags because we are still determining how to group things, but in the long run it should settle down.